Materials Science Forum Vol. 710

Abstract: The microstructure developed during hot deformation is the result of deformation mechanisms such as dynamic recovery and dynamic recrystallization. Hot deformation can also result in damage and flow localisation, especially in multiphase metal based materials. Several models have been proposed to correlate the parameters of the deformation process (temperature, strain and strain rate) with the flow behaviour such as the processing maps. They were developed based on the dynamic materials model (DMM) and later a modified DMM introduced some changes in the calculation of the processing maps. The correlation of the relevant microstructural changes with thermodynamic parameters are tested and discussed. The data was obtained by using the Gleeble simulator with in situ quenching facilities. Microstructural studies related to the hot deformation of metals were carried out based on alpha-beta and near beta titanium alloys and on low carbon steels. The results are correlated with the efficiency of power dissipation, and the constitutive equations. In diffusion controlled processes such as dynamic recovery, dynamic recrystallization, phase transformation and pore coarsening are related to high power efficiency, and to low n exponent. The efficiency of power dissipation is more sensitive to the deformation parameters than the constitutive equations for materials with phase transformation.

Abstract: Material removal, forming, casting and joining are the established manufacturing approaches and processes based on these approaches are being practiced even in modern industries with appropriate automation. Layer by layer material deposition method to produce prototypes from a solid model is relatively new and was developed during last 10-15 years of 20th century. These processes were named as Rapid Prototyping (RP) or Solid Freeform Fabrication (SFF). Today there are many commercial RP system and most of these able to deposit liquid or solid/powder polymer based materials. Some systems are also able to deposit blends of polymer and metal or ceramic. Latest trend in this area is to deposit metals or alloys with variable composition and hence to produce functionally graded material. This paper describes in general the details related to RP processes, data preparation, and various commercial RP technologies. The article also discusses applications these processes.

Abstract: Titanium alloys are used for high-pressure gas bottles / propellant tanks and structural applications owing to their high specific strength, good fabricability / weldability and compatibility with various working fluids. For these applications at ambient temperature, the workhorse Ti6Al4V alloy is extensively used. For the applications at low temperatures, two ELI grades of titanium alloys namely Ti6Al4V and Ti5Al2.5Sn are used as these retain toughness down to 77K and 4K respectively. Due to this inherent advantage, Ti5Al2.5Sn-ELI alloy has been selected as high pressure helium gas bottle submerged in liquid hydrogen (20K temperature). The gas bottle is spherical in shape and is made by electron beam welding of two machined hemispherical shells of 500 mm nominal diameter. The hemispherical shells for the difficult-to-forge Ti5Al2.5Sn-ELI alloy are developed through controlled closed-die forging operations. Shells are subsequently characterized for microstructures and mechanical properties at ambient temperature. Substantial increase in tensile strength with reasonably good ductility with respect to ambient temperature is achieved at 20K temperature. Multi point necking is observed at 20K. The present paper briefly outlines the process control devised for development of these domes and discusses the various characterization results obtained on forged hemispherical shells.

Abstract: The objective of the present study was to melt and cast AA2195 alloy in Vacuum Induction Melting (VIM) under dynamic inert atmosphere. These billets were homogenized and subsequently hot forged and rolled to sheets. The products in the form of sheets were subjected to T8 (Solution Treatment +WQ+CW+Aging) temper condition. Mechanical properties were evaluated at room temperature and correlated with microstructure. Highest mechanical properties obtained in T87 temper have been reported.

Abstract: A novel technique of pure Lithium addition has been adopted for the processing of Al-Cu-Li alloy AA2195 cast ingots (7-8 kg each) in VIM under dynamic inert atmosphere, which gives more than 95% recovery of Lithium. The cast billets were homogenized, forged and converted into 12mm diameter rods by caliber rolling in the temperature range of 250°C, 300°C, 350°C and 400°C. The caliber rolled rods were treated to T8 (Solution Treatment+WQ+CW+Aging) condition. Mechanical properties were evaluated for T8 tempered bars at room temperature and correlated with microstructural observations. Highest mechanical properties in T87 temper have been obtained for rods caliber rolled at 350°C temperature.

Abstract: Magnesium alloys are the most demanded lightweight structural materials for different engineering applications such as aerospace, automobile, electronics, etc. However, the high temperature properties of Mg alloys are not comparable with its competitor, the Al alloys. Mg alloys are not recommended beyond 120°C due to their poor creep and oxidation resistance. In order to improve the high temperature properties of the magnesium alloys, rare earth containing Mg alloys were developed. Among these alloys, Mg-Sc alloys were found to be very interesting which exhibits better high temperature properties. In the present work, magnesium-scandium alloy was fabricated through liquid metallurgy route under inert cover. The alloy was characterized by optical microscopy, X-Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and hardness testing. The microstructural analysis reveals the α-Mg phase and the distribution of fine Mg-Sc intermetallic. It is observed from the DTA that the melting point of the base alloy has got enhanced by the addition of Sc. There is also an appreciable improvement in the hardness by the addition of Sc.

Abstract: Hot working of titanium aluminide made through reaction synthesis (RS) has been studied. Hot isostatic pressing under argon, isothermal pressing under vacuum, open die forging and pack forging was carried out. Hot worked billet samples were characterized for density and microstructure evolution. Presence of refined grains and dynamically recrystallized grains were observed. In case of open die forging, presence of twins was also found. From the present study, near isothermal working through near conventional processes has been confirmed. However, role of isothermal working in obtaining crack free sound deformed product has also been noted.

Abstract: Cu-8Cr-4Nb alloy with high strength and high thermal conductivity have received a lot of attention over the last decades due to promising role in space application of rocket combustion chamber of reusable launch vehicle. In the present work, the copper alloy is synthesized in induction furnace by liquid metallurgy route wherein elemental powders of chromium and niobium was used added into premelted copper melt under argon atmosphere. The cast product is characterised for hardness, optical, SEM, EDS and XRD. It is possible to produce this alloy with reproducible average hardness of 135 VHN. The precipitate of Cr₂Nb showed moderate distribution in and around the grain boundaries of copper with precipitate size varying from 1.33 μm to7.0 μm.

Abstract: Low alloy steels are an attractive option for high performance structural applications due to cost and availability factors. A low carbon low alloyed steel, DMR 249A, was developed for strategic applications by Indian Navy at Steel Melting Shop II, Bokaro Steel Plant (SMS II, BSL). This paper enumerates the detail process variables modified for this development. DMR 249A grade was having the specified gas contents of hydrogen <2 ppm in final product for the avoidance of detrimental phenomenon like “Hair Line Cracks”. So the Hydrogen content of liquid steel was to be less than 3 ppm and this demand vacuum treatment of the steel. In absence of any Degassing unit at SMS II, BSL, making of DMR 249A steel was a challenging task. The hydrogen content of steel was controlled through restriction of hydrogen in input raw materials, control in degree of deoxidation during tapping, slag basicity at ladle furnace, argon rinsing regime and cooling of slabs i.e. control over diffusion of H₂. The steel was produced with the existing infrastructure at Bokaro Steel Plant with various alloying elements and processed suitably to obtain the desired yield strength, toughness and gaseous content. The control was exercised at various stages of steel making and hot strip rolling to enable achievement of a higher degree of consistency in mechanical properties and microstructure. Quality fulfillment was a great challenge without vacuum degasser unit but Bokaro successfully made the steel with 100% customer satisfaction.

Abstract: Aluminium alloy cans are generally used as containers for electrochemical energy systems like capacitors and advanced batteries. Packing efficiency of batteries depends upon their configuration. Containers in prismatic, cylindrical and elliptic-cylindrical shapes are generally used for battery applications. Elliptic-cylindrical shape has the advantage of better heat dissipation and good packing efficiency in battery assembly. Safety device to be provided in the cans requires material in the minimum half hard (H14) condition. In the present work, ellipsoidal cans of Al-Mn alloy were successfully realised by impact extrusion process. Mechanical properties of the cans were analysed through to ensure the adequacy of the process and it has been demonstrated that impact extrusion is a viable process for making cans for batteries with required mechanical properties, for the can in total and for the safety device in particular.